1. Field of the Invention
The present invention relates to a disk system having a plurality of disk drives and, more particularly, to a disk system provided with the capability of adjusting the clock frequency of the clock signal generator contained in each disk drive so as to suppress the electromagnetic noise level radiated from the total disk system.
2. Description of the Prior Art
Electromagnetic noise is generated and radiated from small-scale computers, personal computers, servers, RAID devices and other data storage systems. The radiated electromagnetic noise (EMI) (hereinafter denoted as radiation noise) from such an electronic apparatus is required not to exceed a certain level (for example that specified by the FCC). To suppress radiation noise, efforts have been made with a cut-and-try approach, for example, by identifying the source of radiation noise and removing high frequency components from the waveform of the source signal or changing the arrangement of grounds (ground planes) so as to secure the earth potential.
In Japanese Patent Laid-open No.1994-131073, a prior art method is disclosed. This method concerns a plurality of option boards mounted on a computer system. Each option board is provided with a VCO to shift the clock frequency thereon so that each option board can have a different clock frequency. In this disclosure, however, no references are made to problems characteristic of disk systems and its application to disk systems is not suggested.
In a recent magnetic disk system consisting of a plurality of magnetic disk drives, it is usual that each magnetic disk drive has similar electronic characteristics and therefore generates radiation noise of almost the same frequency. Accordingly, the inventors of the present invention have found that this overlapping results in a considerably high radiation noise level. Note that in many cases, the radiation noise from a magnetic disk drive originates in its operation clock driving the internal electronic circuit and has harmonic frequencies of the clock's fundamental frequency. Also note that disk systems using optical disk media may suffer this problem, too, not peculiar to magnetic disk systems.
Referring to FIG. 3, the following will describe this problem in a magnetic disk system.
Magnetic disk drives 21, 22 and 23 operate from their clocks having the same frequency. Since radiation noise patterns 24, 25 and 26 emitted from the respective disk drives have the same frequency characteristic, they are added up to a high-resolution noise level 27 from the whole magnetic disk system. Enhancing the electromagnetic shielding of the whole system and reducing the radiation noise emitted from each magnetic disk drive have already reached the limits.